1. Field of the Invention
This invention relates to the field of robotic manipulation of objects and, more particularly, to tactile sensors for flexible gripping of such objects.
2. Description of the Related Art
In order for modern industrial robots to truly interact with their environment, they must be able to grasp and manipulate a wide range of objects even when the objects are presented in random fashion. This necessitates the development of powerful sensing and gripping devices. By and large, available industrial robots can only grasp and manipulate a limited range of objects presented in a precise fashion. One important category of sensing is tactile sensing at the gripping device fingers.
Researchers focusing on the design and performance of tactile sensing devices have suggested that such devices should contain distributed arrays of rugged wear-resistant tactile transducers (tactels) mounted to flexible substrates. The building consensus for desirable properties of these tactile sensors suggests that a tactile array should include an active area of 50-200 sensing elements in a 5.times.10 to 10.times.20 array with a separation of tactels in the 1-2 millimeter range to discriminate typical shapes. Detection of a minimum force of 1-5 grams with an upper limit of approximately 1000 grams is also considered appropriate for most light assembly applications. It is suggester that the time for an entire set of sensor data to be collected should be approximately 10 milliseconds (ms) and that the entire sensor data should be updated at a rate of 100 times per second. The sensor's output should be stable over time and it should operate reliably over a wide temperature range in diverse chemical, mechanical, and electrical environments or, in other words, be robust.
Various tactile array sensors, either for commercial use or developed only for research, employ diverse transduction technologies. Indeed, for the most part these sensors have only been developed in experimental form.
Transduction technology, as it applies to tactile sensors, involves the conversion of properties measured through the contact between the sensor and the object into signals to form a tactile image of objects and space. The common transduction technologies include piezoresistive, piezoelectric, capacitive, magnetic, optical, and mechanical.
Piezoresistive transduction typically relies on the changes in electrical conductivity of silicon or carbon-based material as it is stressed by force, temperature, or pressure to produce an image of the object contacted. In general, the sensors developed using this transduction technology exhibit high resolution and sensitivity, linearity, fast response, a wide range of allowable values for the variable to be measured (dynamic range), reasonable cost, and considerable durability. They are heat resistant, easy to fabricate into matrix arrays, and allow for greater conformability to irregular surfaces. If fabricated as matrix arrays, they can be conveniently scanned with row/column addressing methods and the number of wires for an N row.times.M column sensor may be reduced to N+M wires. However, sensors using conductive elastomers suffer from the problems of electrical noise and creep, and their sensitivity, linearity, response, and dynamic range are inferior to those using carbon fibers.
Piezoelectric sensors utilize polymers which generate an electrical charge in response to an applied force or pressure. These polymers also typically exhibit a pyroelectric effect which is the generation of a voltage when a tactel is heated or cooled. Such sensors are small, low cost, conform to irregular surfaces, have high durability, and good resolution and sensitivity. However, under constant load or constant temperature, the sensor output decays to zero. Moreover, since both pressure and temperature changes produce electric charge, it becomes difficult to distinguish between piezoelectric and pyroelectric effects.
Capacitive transduction utilizes a change in the capacitance of parallel plate capacitors due to the application of a pressure or force causing the distance between the plates to change or the dielectric separating the plates to move. Capacitive sensors with an elastic dielectric layer have been considered favorably since the elastic layer has good mechanical properties with high sensitivity to changing pressures. However, capacitive sensors are susceptible to influence by external fields and require specific materials.
Sensors relying on magnetic transduction technology measure contact surface properties by either movement producing a change in magnetic flux or magnetoelastic materials which show a change in magnetic field when stressed. Magnetic sensors are relatively rare although they have a wide dynamic range, linear and fast response, detect shear and torque, and are fairly robust. However, they are susceptible to stray magnetic fields and can only be composed of a limited group of materials.
Optical sensors convert force or pressure into optical signals through the bending of diaphragms or waveguides and allow designs which are invulnerable to electromagnetic interference. Furthermore, the development of optical fiber technology for transmission and solid-state cameras for intensity imaging has led to new tactile sensor designs which are compact, form high spatial resolution images, eliminate electrical interference, and allow separation of the sensor from processing electronics.
Mechanical sensors typically rely on a matrix of sliding probes or pins to contact the object in interest. They are deemed superior to the majority of sensors reviewed thus far in that they can greatly conform to the shape of an object and evenly grasp and support it. In addition, mechanical sensors are simple to construct and are considered robust for harsh environments. However, they experience probe jamming as the object and sensor contact each other and are bulky and achieve lower spatial resolutions due to the presence of mechanical components in the sensor.
The majority of sensors, except mechancial sensors, possess a protective elastomer cover that contacts the object, but does not conform well to many complex shaped surfaces and, thus, such elastomer cover sensors do not securely grasp objects with such complex shaped surfaces. Moreover, the cover must be made thin to reduce the cross-sensitivity that exists between adjacent tactels in the cover.
The thinness of the cover limits the depth of the space sensed (sampling space) while in contact with the object of interest. This requires a high spatial density of tactels and high sensitivity to cover displacement of tactels to generate accurate shape information from the limited volume of the sampling space. Since analog tactels are easily fabricated into dense arrays with high sensitivity to the displacements required to be measured, they are commonly used in elastomer covered sensors. However, the use of analog tactels requires additional electronic components to convert signals from analog to digital output appropriate for storage and processing by a computer. The analog to digital conversion also reduces the frequency of signal transmission of the tactile array and may present a problem for some real-time sensing applications.
It is, thus, an object of the present invention to provide a gripper/sensor system that is simple to fabricate, operate, grasps securely a wide variety of objects, acquires shape information about each object grasped for manipulation purposes, and has commercial value.
The present invention has been disclosed in "Designing a Highly Conformable Tactile Sensor for Flexible Gripping Using a Digital Probe Array", by Glenn M. Friedman (D.Eng. Thesis), Rensselaer Polytechnic Institute, Troy, New York, August, 1994 (hereinafter referred to as "the Friedman Thesis") and this disclosure is largely taken from it. The actual date of submission of the Friedman Thesis for publication was Aug. 15, 1994. References on the related art may be found on pages 118-128 of the Friedman Thesis. In addition, a partial disclosure of some aspects disclosed herein may be found in Flexible Assembly Systems-1992, The 1992 ASME Design Technical Conferences--4th Conference on Flexible Assembly Systems, Scottsdale, Arizona, Sep. 13-16, 1992, edited by A. H. Soni, University of Cincinnatti, The American Society of Mechanical Engineers, 1992, pages 111 to 117.